When I went looking for a word to describe Marcellus' small home at the top of the building, I remembered that Quintus and Flaccus had rented this type of room in Rome in Chapter 18 of the Oxford Latin Course. The line is ibi erat cenaculum, parvum et sordidum. The definition in the margin defines cenaculum as garret or attic room. The Oxford Latin Dictionary adds that they were often let as lodgings.

The Cassell's Latin Dictionary says that a cenaculum literally means 'an eating room', of the same family as cena, dinner and cenare, to dine. And as the 'eating room' was commonly at the top of the building, it came to mean 'the upper story of a house' or 'attic'.

This is the reason that I decided to go with the passive verb vocatur, is called, rather than the more direct est, is.

Perhaps you are asking what a cenaculum is. A small room situated on the top floor of a building is called a cenaculum.